1. Field of the Invention
This invention relates to Inertial Navigation, and in particular to a Fault Tolerant system for Inertial Navigation which identifies and isolates at least one instrument within an Inertial Navigation Unit which shows substantial performance degradation.
2. Description of Related Art
Navigation is the process of spatially directing a vehicle (Land, Sea, or Aircraft) from a first point (or point of origin) to another or second point in space. Navigation is often categorized as either position fixing or dead reckoning. Examples of position fixing include celestial navigation or determining one's position with respect to known objects or landmarks. LORAN and related radar systems, and the newer NAVSTAR GPS (Global Positioning System) are also examples of position fixing Navigation. Generally speaking, dead reckoning is the process of estimating ones position by knowledge of one's previous position, his course, his speed and the elapsed time. This category of dead reckoning includes doppler Radar and Inertial Navigation Systems.
Inertial navigation is yet further unique, in that it is a specialized form of Navigation whereby one is able to travel from point one to point two without the benefit or need of any information external to the vehicle. Inertial Navigation is completely self-contained within the traveling vehicle. Inertial Navigation is independent of its operating environment such as wind, visibility, or aircraft attitude. Because such form of Navigation does not radiate RF energy, it is immune to countermeasures. Inertial Navigation makes use of the basic laws of physical motion first described by Sir Isaac Newton over three centuries ago.
A standard Inertial Navigation System (INS), in actual use, such as aboard commercial or military aircraft, is often comprised of three or more independent inertial navigation units, where each unit is composed of 3 accelerometers and 3 gyroscopes so that each unit alone is fully capable of performing the navigation function. Three INU's (Inertial Navigation Units) or more represents a costly method to assure redundancy and accuracy of navigation. An early attempt to provide redundancy in less than three INU's is U.S. Pat. No. 3,489,004, issued Jan. 13, 1970 to D. H. Barnhill et. al. for a NAVIGATIONAL REFERENCE DEVICE (Honeywell, Inc. Assignee). This patent teaches the use of at least four sensor mechanical gyroscopes in each INU mounted to an attitude reference device base unit. When one of the gyroscopes of the INU is faulty, the remaining gyroscopes may be used to function as an INU. Thus, in this reference '004 patent, redundancy is provided within each INU and independent of other INU systems. Such a scheme might be applied to a Ring Laser Gyroscope instrument package, but ring laser gyroscopes are typically one-degree of freedom devices and these ring laser INU's generally are packaged with only sets of three instruments, three gyroscopes and three accelerometers.
Another redundancy scheme demonstrated heretofore can be found in U.S. Pat. No. 3,680,355 issued Aug. 1, 1972 to Goldstein et. al. for METHOD AND APPARATUS FOR PERFORMANCE MONITORING OF DUAL PLATFORM INERTIAL NAVIGATION SYSTEMS (The Singer Company, assignee). In this '355 patent, two platforms are used to provide the required redundancy; however, each platform uses two, two-degree of freedom mechanical gyroscopes to provide adequate redundancy. This is the equivalent of using at least four ring laser gyroscopes and again would be costly.
What is needed is a fault-tolerant navigation scheme which conforms to modern Inertial Navigation Unit packages, comprising three gyroscopes and/or three accelerometers, and uses less than three of such Inertial Navigation systems.